1. Field of the Invention
This invention relates to a current supply control method for a line thermal head, and more particularly relates to a current supply control method for a line thermal head that is preferably used for color printing.
2. Description of the Related Art
Heretofore, a thermal transfer line printer has been known, in which a line thermal head having a sufficient length for facing to the printing range in a vertical direction or a horizontal direction of a paper is pressed on a platen roller with interposition of an ink film such as ink ribbon or ink sheet and a paper, the platen roller is driven rotationally in a pressed state, a plurality of heating elements of the line thermal head are driven and heated selectively based on printing information while the ink film and the paper are being moved, and ink of the ink film is thereby transferred and printed on the paper.
In the case that a thermal transfer line printer as described hereinabove is used for color printing, a color ink film that is called multicolor ink film on which at least three color inks of three primary colors, namely Y (yellow), M (magenta), and C (cyan) are disposed repeatedly along a moving direction of a paper is used for color printing. In detail, at first Y ink of the color ink film is printed on the paper over one page (one screen), subsequently a paper that has been moved forward in printing operation is moved backward to align a head of the paper at the printing position, a head of M ink of the color ink film is searched next and M ink of the color ink film is printed on the paper over the one page (one screen), and C ink of the color ink film is printed on the paper in the same manner. Thus, three color inks, namely Y, M, and C, are printed one on another on the paper for color printing.
A line thermal head has a plurality of heating elements arranged in a direction perpendicular to the moving direction of a paper and the number of heating elements is as many as, for example, about 500. As the result, if a current supply control method in which a current is supplied to all the heating elements simultaneously is employed, then the driving circuit is inevitably large and the supply current is also inevitably large to result in difficulty in battery driving. The method as described hereinabove is involved in various problems.
To solve the above-mentioned problems, another current supply control method is used, in which heating elements are divided into groups by grouping with some heating element intervals and a current is supplied to a group at a time so that the number of heating elements to be heated at a time is reduced in order to reduce the magnitude of the current and to miniaturize the driving circuit. Thus, a battery having a small power source capacity can be used for driving.
For example, in the case of 5-divided driving in which a current is supplied to heating elements HD for one line printing, a plurality of heating elements HD arranged in a printing line direction are numbered from a left end, which is the reference, to a right end with numbers 1, 2, 3, 4, 5, 6, . . . in square frames as shown in FIG. 4, a current is supplied to the heating element HD at the left end and heating elements HD positioned at intervals of five heating elements HD from that HD at the left end (heating element numbers 1, 6, 11, . . . ) at first time for driving (current supply order number 1 is given to these heating elements HD in FIG. 4), and a current is supplied to the second heating element HD positioned rightward next to the left end heating element HD and heating elements HD positioned at intervals of five heating elements HD from the second heating element HD (heating element numbers 2, 7, 12, . . . ) at second time for driving (current supply order number 2 is given to these heating elements HD in FIG. 4). Next, a current is supplied to the heating element HD positioned at the third position from the left end in the right direction and heating elements HD positioned at intervals of five heating elements HD from the third heating element HD (heating element numbers 3, 8, 13, . . . ) at third time for driving (current supply order number 3 is given to these heating elements HD in FIG. 4), and a current is supplied to the heating element HD positioned at the fourth position from the left end in the right direction and heating elements HD positioned at intervals of five heating elements HD from the fourth heating element HD (heating element numbers 4, 9, 14, . . . ) at fourth time for driving (current supply order number 4 is given to these heating elements in FIG. 4). Finally, a current is supplied to the heating element HD positioned at the fifth position from the left end in the right direction and heating elements HD positioned at intervals of five heating elements from the fifth heating element HD (heating element numbers 5, 10, 15, . . . ) at fifth time for driving (current supply order number 5 is given to these heating elements HD in FIG. 4). When the next line is to be printed, the heating elements that have been driven at first time (heating element number 1, 6, 11, . . . ) are driven at first again, and other heating elements HD are driven successively in the same manner as described above.
Furthermore, in printing in the case of 5-divided driving, as shown in FIG. 5, a head position of a printing dot PD of the current supply order numbers 1, 2, 3, 4, and 5 is positioned in parallel in a line direction and is displaced by ⅕ one printing dot PD every current supply order number.
Furthermore, the current supply order to the heating element HD is set independently of the ink color, and printing dots of YPD, MPD, and CPD of three color inks Y, M, and C are formed on the same positions, in detail, positions of the current supply order numbers of 1, 2, 3, 4, and 5 in FIG. 5.
However, in the case of the conventional current supply control method described above, the current supply order to the heating element HD is set independently of ink color, that is, the current supply order is set so that the position of the heating element HD to be served for divided driving is independent of the color served for printing. Therefore, to print printing dots YPD, MPD, and CPD of three colors Y, M, and C on the same position on a paper, for example, the printing position accuracy that is so high as positional deviation of a paper that occurs when each color is printed is equal to or less than 5 xcexcm is required in the case of a 300 dpi line thermal head. The reason is that, for example, if the printing position of a printing dot CPD deviates from the printing position of the printing dots YPD and MPD formed by printing Y color ink and M color ink due to the positional deviation of the paper that occurs when C color ink is printed, then the printing position of C color printing dots CPD deviates on all the positions of the current supply order numbers 1, 2, 3, 4, and 5, and the hue tends toward C color as a whole and it tends to cause irregular color and jitter.
Therefore, the conventional current supply control method is involved in the problem that the high mechanism accuracy for supplying a paper with high accuracy is required in order to obtain high printing quality with less irregular color and jitter that associate with the paper feeding accuracy.
To solve such problem, a current supply control method that is capable of easily reducing irregular color and jitter due to the paper feeding accuracy has been expected to be developed.
The present invention has been accomplished in view of these points, it is an object of the present invention to provide a current supply control method used for a line thermal head that is capable of reducing irregular color and jitter easily and surely.
To achieve the above-mentioned object, a current supply control method used for a line thermal head in accordance with the present invention is characterized in that the heating element to be served for divided driving is controlled so as to be different for each color to be printed. Because each heating element is served for printing correspondingly to the number of divisions and the printing position of the printing dot of each color is formed at the position with deviation by employing the structure described above, the irregular color and jitter due to deviation of paper feeding position are made unremarkable. Therefore, the adverse effect of deviation of paper feeding position is mitigated, and the irregular color and jitter due to the paper feeding accuracy is reduced easily.
The current supply control method used for a line thermal head in accordance with the present invention is characterized in that the number of divisions of the divided driving is an odd number. The number of divisions that is suitable for easily reducing the irregular color and jitter due to the paper feeding accuracy is obtained by employing the structure described above.
Furthermore, the current supplying control method used for a line thermal head in accordance with the present invention is characterized in that the number of divisions of the divided driving is 5. The number of divisions that is suitable for easily reducing the irregular color and jitter due to the paper feeding accuracy is obtained by employing the structure described above.